THREAD FACTS

The following will give you information on the threads so they can be better understood. Thread cutting
on the lathe is the most common way to make screws, etc. in the shop. Just about any type of screw,
and thread that is needed in the shop, can be made on a lathe.

The major or outside diameter (O.D.) of an external thread is the diameter of the piece on which the
thread is cut. LTL is the largest diameter of the thread.

The depth of a thread is the distance from the top or crest of the thread to the root measured vertically.

The minor diameter is the smallest diameter of the thread of the screw. LTL is sometimes called the
root diameter (R.D.) and can be found by subtracting twice the depth of the thread from the major
diameter.

The number of threads per inch (N) can be counted by placing a rule against the threaded part, and
counting the threads in one inch. The first thread is not counted since, in reality, not the crests but the
spaces between the crests are what are being counted. A second method is to use a screw pitch gage.
This method is more suitable for checking the finer pitches of screw threads.

The pitch (P) of a thread is the distance from a point on a screw thread to a corresponding point on the
next thread, measured parallel to the work axis. The pitch of a thread in inches can be found by dividing
the whole number 1 by the number of threads per inch (N).

On a screw thread, the pitch diameter is that of an imaginary cylinder. The outer surface of this cylinder
would pass through the threads at such points as to make equal the width of the threads and the width of
the spaces. On a 60-degree V type thread and on National form threads, the pitch diameter can be
found by subtracting the single depth of the thread from the major diameter of the thread.

The lead of a thread is the distance a screw will advance into a nut in one complete revolution. The lead
is the same as the pitch on a single-thread screw.

The formula for the lead of a single thread screw, then, is:

1 P (in inches)

N (per inch)

The lead for a screw with nine threads per inch is .1111, which is the same as the pitch in inches
(.1111).

On a double-thread screw, the lead is twice the pitch. On a screw with nine threads per inch with a
double-thread screw the lead would be doubled or .2222. On a triple-thread screw, the lead is three